As of December 1979, 153 patients have received elective postoperative radiotherapy, with moderate doses, for a locally advanced colorectal cancer (Stage B2, C1, C2). Ninety had rectosigmoid cancer and 55 a colonic lesion. 125 patients had a minimum followup of twelve months. The absolute survival, with a mean followup of thirty months, of patients with B2 and C1 lesions is about 70%. That of C2 is 55%. The absolute survival at 24 and 36 months for the entire 126 patients is 69% and 53% respectively. 83% and 79% of patients are recurrence free for the same period of time. Thirty-one patients had residual disease left behind following the surgical resection. Twenty (65%) are locally controlled. Radiation complications occurred in six of the 125 patients (6%). Only one patient (0.8%) had a fatal complication. We are continuing to study the effect of large-field irradiation on the immune status of these patients. So far, we have documented the suppression of mitogenic response from non-specific mitogens and the decrease in the number of peripheral T-lymphocytes. Also, interestingly, there is an increase in the number of activated monocytes following radiotherapy which may be due to the release of immune complexes. From our data, it appears that postoperative radiotherapy should be given routinely, following the surgical resection of a locally advanced colorectal cancer.